Ch 25 - Plant Hormones and Plant Growth I. Patterns of plant growth A. Plant continue to grow, even in old age. i.e. new leaves, needles, new wood, new cones, new flowers, etc. B. Meristem continues to make new cells C. Plants grow in response to environmental factors 1. light 2. Moisture 3. Temperature 4. Gravity II. Plant Hormones A. Definition - Chemical substances that control a plant s pattern of growth and development.
-2- B. Response os triggered in hormone producing cells C. Response affects the target cells or tissue. D. Target cells contain receptors to change metabolism, affect growth rate, etc E. One hormone may have different responses in different cells - ex. growth in stems, but inhibit root growth. III. Phototropism - plant grows toward light Draw and label fig. 25-3 p.635
-3- A. Darwin s experiments showed plants bend toward the light B. Auxins move into the side of a plant opposite light source C. Cells elongate (opposite side of sun) causing the plant to bend to the light D Auxins 1. Gravitropism a. Growth response to gravity b. Mechanism is the same as in phototropism 2. Branching and apical dominance a. Lateral buds are inhibited by auxins at first in the young plant
-4- b. Plants will continue to grow tall until the tip of the plant is removed c. Removal causes lateral buds to grow 3. Auxin like weed killers a. High concentrations of auxin inhibit growth b. Herbicides - 2,4-D, Agent Orange E. Cytokinins 1. Stimulate lateral bud growth and dormant seeds will sprout 2. Produces opposite effect to auxin 3. Relative concentrations of cytokinins and auxin determines how a plant will grow
-5- F. Gibberellins 1. Discovered in Japan as a fungus that causes rice to grow unusually tall (bad for rice) 2. Produce dramatic increase in size of stems and fruits 3. Responsible for rapid early growth of plants G. Ethlene 1. Component of natural gas that causes trees to lose their leaves and stunt growth 2. Plants produce ethylene in very small amounts 3. Main function is to stimulate f ruit production - ripening
-6- VI. Plant responses - plant behavior A. Tropisms - a plants ability to respond to its environment 1. Geo - gravitropism a. plant s response to gravity b. Causes shoot to grow out of soil - against gravity c. Caused roots to grow into the soil - with gravity d. Auxin controlled 2. Phototropism a. Plants response to the sun or a light source b. Causes plant tot grow toward the light c. Auxin controlled
-7-3. Thigmotropism a. Response of the plant to touch b. Regular touching plants will stunt growth c. Climbing vines 1). Stem of these plants grow in circles 2). When meeting an object it wraps around tightly 3). Example: grapevine tendrils B. Rapid Responses 1. Minosa pudica a. Closes leaf in 1-2 seconds after being touched
-8- b. Osmotic pressure is rapidly decreased - leaf closes (wilts) c. See text p. 640 fig 25-9 2. Venus Fly Trap is carnivorous a. Fly triggers sensory cells to send signals from cell to cell b. The combination of changes snaps the leaf shut with insect inside VII. Photoperiodism - Seasonal flowering of certain plant A. Chrysanthemums and Poinsettias bloom only in fall (short day, long night) B. Spinach and Irises bloom only in spring (long day, short night)
-9- C. Short day plants - flower after a critical period of night usually longer than 12 hours D. Long day plants - flower after a critical period of night usually shorter than 12 hours E. Day neutral plants - night or light length has no effect F. Phytochrome is a plant pigment that absorbs red light waves and signals pathways in the plant for flowering G. Study fig 25-10 p. 641 If the critical night period is interrupted by a short burst of light, phytochrome absorbs the light and causes a short day plant to bloom.
-10- VIII. Winter Dormancy - Regulated by phytochrome A. Defined as an organism growth or activity decreases or stops B. Colder temperatures reduces photosynthesis efficiency C. Green chlorophyl gradually is lost and yellow and orange and red chlorophyl is left, which is also lost D. Plants go dormant to withstand the cold winter E. Leaf abscission - abscission layer allows leaf to fall
-11- IX. Plant Adaptations A. Aquatic plants 1. survive because they have evolved specialized air spaces in petioles and stems 2. Air spaces trap oxygen and transport it to roots 3. Examples include: Mangroves, Water Lilies and Cypress Trees B. Salt Tolerant Plants 1. Specialized cells pump the excess salt out of the plant tissues onto the leaves to be washed away by rain 2. Example: Mangroves
-12- C. Desert Plants - Xerophytes - plant adaptations include extensive root system, reduced leaves and thick stems X. Epiphytes - Parasites A. Plants that grow on other plants B. Examples: - Spanish Moss (not a moss) found in warm regions - Witch Hazel - deciduous forests - Orchids - tropics XI. Chemical defenses A. Some plants produce chemicals that are poisonous to most animals and insects
-13- B. Some plant chemicals have been refined by man to produce medicines Examples: 1. Digitalis comes from Foxglove - poisonous when eaten, but when refined can help cure heart disease 2. Milkweed - toxic to most animals, but the Monarch butterfly is unaffected..